star trek - Why are Romulans using warp scale?

Warp is never defined on screen. There is no explanation of it, or why the scale changes between TOS and TNG+. There are only a few benchmarks of how fast a certian warp factor is; warp 1 is the speed of light, warp 10 is everywhere at once, warp 5 is "here to Uranus and back in 10 minutes" or some such. There is nothing in the series that states that it is a Federation, Human, or natural unit (a universal constant).

Knowing this, one possibility is that Warp is just the English word for a universal constant, where the Romulan unit of kinetic energy is not a natural unit (neither is Celsius) and is not trivially translated.